Fano effect in the a-b plane of Nd1.96Ce0.04CuO4+y: evidence of phonon interaction with a polaronic background

Reflectance measurements in different Nd1.96Ce0.04CuO4+y samples with properly selected carrier concentrations provide firm evidence of Fano antiresonances for the four transverse optical E-u phonons in the a-b plane. An analysis of the Fano line shapes and of the dependence on temperature of the renormalized phonon frequencies allows us to determine the origin of the electronic continuum interacting with the phonons. Unlike in doped semiconductors. where the continuum is the free-carrier absorption band, here the continuum is provided by a polaron band at similar to 0.1 eV, present in most parent compounds of high-T-c superconductors. In the most doped sample, the polaron band softens as T decreases, thus indicating a delocalization of the polaronic carriers at low temperature.